Multilayer nacreous pigment

ABSTRACT

Multilayer pearl lustre pigment on the basis of a platelet-shaped substrate comprising a material of low refractive index in the range from 1.35 to 1.8, which comprises at least 
     (i) one first layer comprising a material having a refractive index of more than 1.8, 
     (ii) optionally, a second layer comprising a material of low refractive index in the range from 1.35 to 1.8, 
     (iii) a semitransparent metal layer which is applied to the substrate or to the layers (i) or (ii), and 
     (iv) if desired, an aftercoating, the substrate being platelet-shaped silicon dioxide, aluminium oxide, boron oxide or magnesium fluoride.

The invention relates to a multilayer pearl lustre pigment having apronounced colour flop, based on a platelet-shaped substrate comprisinga material of low refractive index in the range from 1.35 to 1.8.

Multilayer pigments which exhibit an angle-dependent colour changebetween two or more intensive interference colours are known.

For instance, U.S. Pat. No. 4,434,010 describes a multilayerinterference pigment consisting of a central layer of a reflectivematerial (aluminium) and alternating layers of two transparent,dielectric materials of high and low refractive index, for exampletitanium dioxide and silicon dioxide, on either side of the centralaluminium layer. In a further embodiment of the pigment, the layersfollowing the central aluminium layer are formed by magnesium fluorideand chromium. This pigment exhibits an intensive colour flop from greento purplish red.

EP 0 753 545 describes goniochromatic lustre pigments based ontransparent, non-metallic, platelet-shaped substrates, which have atleast one layer stack comprising a colourless coating with a refractiveindex n≦1.8 and a reflective, selectively or non-selectively absorbingcoating which is at least partly transparent to visible light, and whichalso have, if desired, an external protective layer in addition.

These pigments have the disadvantage that they are produced by atechnically very complex and costly process, for example by chemicalvapour deposition (CVD) or physical vapour deposition (PVD) techniques.Further disadvantages are the frequent difficulty in reproducing thepigments in the desired product quality, and their deficient weatheringstability.

It is the object of the present invention to provide an essentiallytransparent interference pigment having strong interference coloursand/or a high angular dependency of the interference colours andfeaturing advantageous applications properties, which at the same timeis simple to produce.

This object is achieved in accordance with the invention by a multilayerpearl lustre pigment on the basis of a platelet-shaped substratecomprising a material of low refractive index in the range from 1.35 to1.8, which comprises at least

(i) one first layer comprising a material having a refractive index ofmore than 1.8,

(ii) optionally, a second layer comprising a material of low refractiveindex in the range from 1.35 to 1.8,

(iii) a semitransparent metal layer which is applied to the substrate orto the layers (i) or (ii), and

(iv) if desired, an aftercoating.

If the semitransparent metal layer forms the outer layer of the pigment,it is also possible for layers of high and low refractive index tofollow. Before the metal layer is applied, the first and second layersmay also be repeated.

This object is further achieved, in accordance with the invention, by aprocess for producing the pigment of the invention by

applying a precursor of the substrate material as a thin film to asmooth surface,

solidifying the liquid film by drying,

detaching the dried film and treating it, if desired, with an acid,

washing the resultant substrate particles and resuspending them in acoating solution,

coating the substrate particles with two or more layers of metal oxidesor metals, and

aftercoating the resultant pigment.

In addition to the purely colouristic applications, the pigments of theinvention can also be considered for functional applications. Examplesof these are as pigments for the security sector, e.g. the printing ofitems of value and of security, as pigments with specific IR reflection,e.g. for glasshouse films, and as pigments for the laser marking ofplastics.

The invention additionally provides for the use of the pigments of theinvention in paints, varnishes, printing inks, plastics, ceramicmaterials, glasses and cosmetic formulations. For these purposes theymay also be employed as mixtures with commercially customary pigments,examples being organic and inorganic absorption pigments, metal-effectpigments and LCP pigments.

The pigments of the invention are based on platelet-shaped substrates.These substrates may consist of silicon dioxide, silicates, boron oxide,borates, aluminium oxide, glass, magnesium fluoride or other transparentand stable materials capable of taking on soluble or insolublecolorants.

Precursors employed for the production of the substrates are solutionsof organic or inorganic compounds of the metals aluminium, silicon,potassium or sodium with borates, aluminates, poly- and/ormetaphosphates, silicates or mixtures thereof. A preferred precursor iswaterglass.

The platelet-shaped substrate particles have a thickness of between 0.05and 5 μn and, in particular, between 0.2 and 2 μm. The extent in theother two dimensions is between 1 and 250 μm, and, in particular,between 2 and 100 μm.

Insoluble colorants incorporated into the substrate may be pigmentparticles whose dimensions are markedly smaller than the thickness ofthe substrate. The particle size of the commercially customary pigmentsmust therefore be adapted to the desired layer thickness of thesubstrate. The term pigment particles here should be interpreted broadlyand embraces white, black, colour and fluorescent pigments.

Suitable inorganic pigments are white pigments such as titanium dioxide,barium sulphate or zinc oxide, examples being Titandioxid 2310(manufacturer: Kronos), Titandioxid R-D (manufacturer: Bayer) andTitandioxid R-506 (manufacturer: Sachtleben).

Suitable black pigments are magnetite or pigment black, an example beingFarbrub FW 200 (Degussa).

Suitable colour pigments are iron oxide or chromium oxide, mixed-phaseoxides such as (Ti, Cr, Sb)O₂, CoAl₂O₄ (Thenard's Blue), ZnAl₂O₄(Rinman's Green), (Fe,Cr)₂O₃ and also sulphides, for example CdS.

Also suitable are inorganic fluorescent pigments, such as fluorescentsilver-doped zinc oxide, phosphorescent copper-doped zinc sulphide, orultramarine pigments.

Suitable organic pigments are azo pigments, anthraquinone pigments,indigo or thioindigo derivatives, diketopyrrolopyrrole pigments,perylene pigments or phthalocyanine pigments. Particularly suitable redpigments are Paliogen Maron L3920 (manufacturer: BASF), DPP-Irgazin RedBO (manufacturer: Ciba), Chinquaisia Margenta RT355D (manufacturer:Ciba), Hostaperm Red E2B70 (manufacturer: Hoechst—Clariant), SicotransRed L2817 (manufacturer: BASF), Carmine Red, Thioindigo, DC Red 6, alsoknown as Lithol Rubin 13, and DC Red 33, also known as Acid Fuchsine.Particularly suitable blue pigments are Hostaperm Blue AFL(manufacturer: Hoechst—Clariant), Irgazin Blue A3RN (manufacturer:Ciba), Paliogen Blue L6470 (manufacturer: BASF), Prussian Blue, and FDCBlue 1, also known as Brilliant Blue. Particularly suitable greenpigments are Monastral Green 64 Special (manufacturer: Zeneca—ICI),Hostaperm Green 8G (manufacturer: Hoechst—Clariant), DC Green 5, alsoknown as Alizarin Cyanin Green F and particularly suitable yellowpigments are Irgazin Yellow 5GTL (manufacturer: Ciba), Irgacolor Yellow2GLMA (manufacturer: Ciba), FDC Yellow 5, also known as tartrazine, andFDC Yellow 6, also known as Sunset Yellow.

To aid dispersion of the pigment particles in the precursor it is inmany cases advantageous to add wetting agents, for example Hydropallat884 (manufacturer: Henkel). Neither the type nor the amount of thewetting agent added is critical, although in general the proportion ofwetting agent is not more than 2% by weight, based on the dispersion.

The weight fraction of the incorporated pigment particles, based on theweight of the uncoated substrate, is between 0.5 and 40% and, inparticular, between 5 and 25%. Further details can be found in EuropeanPatent 0 608 388.

The substrate may alternatively comprise a soluble colorant as thecolorant. The term “soluble colorant” means either a chromophoric metaloxide, for example iron oxide, chromium oxide or cobalt oxide, or asoluble organic dye.

Generally suitable for the colouring of the substrate are chromophoriccompounds of the metals titanium, vanadium, chromium, manganese, iron,cobalt, nickel and copper, preferably compounds of cobalt, copper, ironand chromium. They are added as soluble compounds to the precursor ofthe substrate. The result is a coloured, transparent substrate having acolour scale similar to that of coloured, transparent glass. Adding ironcompounds, for example, gives reddish brown shades, adding chromiumcompounds green shades, and adding cobalt compounds blue shades.

Soluble organic dyes which can be employed include alkali-solublehydroxyanthraquinone dyes or acidic azo dyes.

The soluble colorant is present in the uncoated substrate in aproportion of from 0.01 to 50% by weight, preferably from 1 to 30% byweight. Further details can be found in EP 0 608 388.

Suitable layer material for the layer having a refractive index of morethan 1.8 comprises all materials of high refractive index which areknown to the person skilled in the art and can be applied permanentlyand in film-like manner to the substrate particles. Particularlysuitable are metal oxides or metal oxide mixtures, such as TiO₂, Fe₂O₃,ZrO₂, ZnO, SnO₂, or compounds of high refractive index, such as irontitanates, iron oxide hydrates, titanium suboxides, chromium oxide,bismuth vanadate, cobalt aluminate, and also mixtures and/or mixedphases of the said compounds with one another or with other metaloxides. Metal sulphides, metal nitrides and metal oxynitrides are alsosuitable.

The thickness of the layer (i) is 10-550 nm, preferably 15-400 nm and,in particular, 20-350 nm. Suitable layer materials for the layer of lowrefractive index (ii) are preferably metal oxides and/or thecorresponding oxide hydrates, such as SiO₂, Al₂O₃, AlO(OH), B₂O₃ or amixture of the said metal oxides, or MgF₂. The thickness of the layer is10-1000 nm, preferably 20-800 nm and in particular, 30-600 nm.

Alternatively, the material of low refractive index employed cancomprise polymers, such as acrylates. The monomers used have a molecularweight of from 200 to 1000 and are available as mono-, di- ortriacrylates. In terms of functional groups, they are available ashydrocarbons, polyols, polyethers, silicones or fluorinated Teflon-likemonomers. These monomers carl be polymerized by electron beams or UVrays. The layers obtained possess a temperature stability of up to 250°C. The refractive indices of the acrylate layers lie within the rangefrom 1.35 to 1.60. Further details can be found in David G. Shaw andMarc G. Langlois: Use of a new high speed acrylate deposition process tomake novel multilayer structures, MRS Conference in San Francisco 1995;A new high speed process for vapour depositing fluoro and siliconeacrylates for release coating applications, Conference of the Society ofVacuum Coaters in Chicago, Ill., 1995.

The layer thickness of the polymer layer is set at values between 20 and700 nm, preferably between 60 and 500 nm.

The metal layers (iii) consist of metals, such as aluminium, chromium,nickel, chromium-nickel alloys or silver. Chromium and aluminium arepreferred here, since they are easy to deposit. The layer thickness ofthe metal layers is set at from 5 to 20 nm in order to obtainsemitransparency. Alternatively, materials such as graphite or titaniumnitride can be employed as semitransparent reflector layers.

The pigments of the invention also include additional colorants in themetal oxide coating. If, for example, particles of carbon black areused, then particle sizes of from 5 to 200 nm, and, in particular, from10 to 100 nm are used. Pigments of this kind, which contain preferablycarbon black particles in layers of titanium dioxide, iron oxide, tinoxide, chromium oxide and zinc oxide, are described in EP 0 499 864.

In addition, the pigments of the invention may also comprise particlesof titanium dioxide, aluminium oxide, silicon dioxide, tin dioxide,magnesium oxide, zinc oxide, cerium dioxide, tungsten oxide, molybdenumoxide, zirconium oxide, or else mixed oxides, such as Cr₂FeO₄, CoAl₂O₄or NiAl₂O₄, in the metal oxide layer.

Instead of inorganic pigment particles it is also possible for organicpigment particles to be present in the metal oxide layer, in which caseparticular preference is given to temperature-stable organic pigments.Organic pigment particles used are preferably phthalocyanines, productsof laking basic dyes with heteropolyacids, and anthraquinones,phenazines, phenoxazines, diketopyrrolopyrroles or perylenes. Inprinciple, all pigments which have been described for incorporation intothe substrate can also be incorporated into the coating of the pigmentof the invention. The incorporation of small particles of metal oxide ororganic pigment having an average size of from 10 to 40 nm into thecavities of the metal oxide coating brings about a marked increase inthe hiding power and in the lustre, in association with a high level ofhomogeneity of the coating in comparison to pigments obtained bycoprecipitation. The hiding power and, in the case of coloured pigmentparticles, the observation-angle-dependent absorption colour of thepigments of the invention can be varied within a wide range by way ofthe concentration of the pigment particles incorporated. The massfraction of incorporated pigment particles, based on the coating, liesbetween 0.5 and 30% and, in particular, between 2 and 20%. Furtherdetails of pigments which comprise pigment particles in the coating canbe found in DE 41 40 295.

The finished pigment is subjected to an aftercoating or aftertreatment(iv), which increases further the light stability, weathering stabilityand chemical stability, or which facilitates the handling of thepigment, especially its incorporation into various media. Suitableaftercoatings or aftertreatments are, for example, the processesdescribed in DE-C 22 15 191, DE-A 31 51 354, DE-A 32 35 017 or DE-A 3334 598.

The additionally applied substances account for only from about 0.1 to5% by weight, preferably from about 0.5 to 3% by weight, of the overallpigment.

The number and thickness of the layers is dependent on the desiredeffect and on the substrate used. The number of layers is limited by theeconomics of the pigment. If the substrate used is SiO₂ platelets, whichin accordance with the process described in EP 0 608 388 are produced ona continuous belt, it is possible to obtain particularly well-definedinterference effects, since in contrast to mica these SiO₂ plateletspossess a uniform layer thickness. The reflection spectrum ortransmission spectrum of such a pigment features finer and moreprecisely harmonizable structures than the spectrum of a correspondingpigment which is based on a substrate having a broad thicknessdistribution, such as mica, for example.

In accordance with EP 0 608 388 the SiO₂ platelets are produced on acontinuous belt by solidification and hydrolysis of a waterglasssolution.

The metal oxide layers are preferably applied by wet-chemical means, itbeing possible to employ the wet-chemical coating techniques developedfor the production of pearl lustre pigments; such techniques aredescribed, for example, in ,DE 14 67 468, DE 19 59 988, DE 20 09 566, DE22 14 545, DE 22 15 191, DE 22 44 298, DE 23 13 331, DE 25 22 572, DE 3137 808, DE 31 37 809, DE 31 51 343, DE 31 51 354, DE 31 51 355, DE 32 11602, DE 32 35 017 or else in further patent documents and in otherpublications.

For coating, the substrate particles are suspended in water and thesuspension is admixed with one or more hydrolysable metal salts at a pHsuitable for the hydrolysis, this pH being chosen such that the metaloxides and/or metal oxide hydrates are deposited directly on theparticles without instances of secondary precipitation. The pH isnormally held constant by simultaneous metered addition of a base.Subsequently, the pigments are separated off, washed and dried and, ifdesired, calcined, it being possible to optimize the calcinationtemperature in respect of the particular coating present. If desired,the pigments can be separated off, dried and, if desired, calcinedfollowing the application of individual coatings, before then beingresuspended in order to apply the further layers by precipitation.

In addition, coating can also be carried out by gasphase coating in afluidized-bed reactor, it being possible to employ, accordingly, thetechniques proposed in EP 0 045 851 and EP 0 106 235 for the productionof pearl lustre pigments.

For the application of titanium dioxide layers, preference is given tothe technique described in U.S. Pat. No. 3,553,001.

An aqueous titanium salt solution is added slowly to a suspension,heated to about 50-100° C., especially 70-80° C., of the material to becoated, and a substantially constant pH of about 0.5-5, in particularabout 1.5-2.5, is maintained by simultaneous metered addition of a base,such as aqueous ammonia solution or aqueous alkali metal hydroxidesolution, for example. As soon as the desired layer thickness of theTiO₂ precipitate is reached, the addition of the titanium salt solutionand of the base is stopped.

This technique, which is also referred to as the titration technique, isnotable for the fact that it avoids an excess of titanium salt. This isachieved by supplying to the hydrolysis per unit time only that quantityof titanium salt solution which is required for uniform coating with thehydrated TiO₂ and can be received per unit time by the available surfacearea. Consequently, no hydrated titanium dioxide particles are producedthat are not precipitated on the surface to be coated.

For the application of the silicon dioxide layers, the followingtechnique can be employed: a sodium waterglass solution is metered intoa suspension, heated to about 50-100° C., especially 70-80° C., of thematerial to be coated. The pH is held constant at from 4 to 10,preferably from 6.5 to 8.5, by simultaneous addition of 10% hydrochloricacid. Following the addition of the waterglass solution, stirring iscontinued for 30 minutes.

The individual layers can also be produced in accordance with knowntechniques by sputtering metals, such as aluminium or chromium, oralloys, such as Cr—Ni alloys, and also metal oxides, for exampletitanium oxide, silicon oxide, or indium-tin oxide, or by thermalevaporation of metals, metal oxides or acrylates. Preference is given toa vacuum belt coating as discribed in DE 197 07 805 and in DE 197 07 806for the production of interference pigments.

What is claimed is:
 1. A multilayer pearl lustre pigment comprising aplatelet-shaped substrate substantially of a material having a lowrefractive index in the range from 1.35 to 1.8, and selected fromsilicon dioxide, aluminium oxide, boron oxide or magnesium fluoride,having thereon: (i) one first layer comprising a material having a highrefractive index of more than 1.8, (ii) optionally, a second layercomprising a material of a low refractive index in the range of 1.35 and1.8; and (iii) a semi-transparent metal layer which is applied to thesubstrate or to the layers (i) or (ii).
 2. A pearl lustre pigmentaccording to claim 1, wherein the material of high refractive index isTiO₂, ZrO₂, Fe₂O₃, Cr₂O₃, ZnO or a mixture of these oxides or an irontitanate, an iron oxide hydrate, a titanium suboxide or a mixture and/ormixed phase of these compounds.
 3. A pearl lustre pigment according toclaim 1, wherein the material of low refractive index is SiO₂, Al₂O₃,AlOOH, B₂O₃, MgF₂ or an acrylate, and optionally, alkali metal oxides oralkaline earth metal oxides present as additional constituents.
 4. Aprocess for producing the pigment according to claim 1, comprising:applying a precursor of the substrate material as a thin film to acontinuous belt, solidifying the liquid by drying, detaching the driedfilm and treating it, optionally with an acid, washing the resultantsubstrate particles and resuspending them in a coating solution, andcoating the substrate particles with at least two or more layers ofmetal oxides or metals, with the proviso that at least one layer has ahigh refractive index of more than 1.8 and at least one layer is asemi-transparent metal layer.
 5. A process according to claim 4, whereinthe precursor employed is a solution of organic or inorganic compoundsof the metals aluminum, silicon, potassium or sodium with borates,chlorides, aluminates, poly- and/or meta-phosphates, silicates ormixtures thereof.
 6. A process according to claim 4, wherein theprecursor is waterglass.
 7. A process according to claim 4, whereincolorants are added to the precursor, either the colorant beingdispersed or dissolved in the precursor prior to application to thebelt, or the components being applied separately to the belt by way oftwo or more nozzles.
 8. A process according to claim 4, wherein prior toor during application to the continuous belt, particles of an organic orinorganic pigment having dimensions smaller than the thickness of thesubstrate, are dispersed in the precursor.
 9. A process according toclaim 8, wherein the dispersed amount of pigment particles is from 0.01to 99% by weight, based on the precursor.
 10. A process according toclaim 4, wherein following drying of the substrate to be coated thelayers are applied in a fluidized-bed reactor by CVD and/or PVD.
 11. Apaint, printing ink, plastic, cosmetic, ceramic, glass or polymer filmpigmented with a pigment according to claim
 1. 12. A laser-markableplastic comprising a pigment according to. claim
 1. 13. A pearl lustrepigment according to claim 1, wherein the pigment additionally containsan aftercoating or aftertreatment, which increases the stability orfacilitates the handling of the pigment.
 14. A process according toclaim 4, wherein the pigment is additionally aftercoated oraftertreated, thereby increasing the stability or facilitating thehandling of the pigment.
 15. A process according to claim 9, wherein thedispersed amount of pigment particles is from 1 to 30% by weight, basedon the precursor.
 16. A pearl lustre pigment according to claim 1,wherein the platelet shaped substrate has a thickness between 0.05 and 5μm and an extent in the other two directions between 1 and 250 μm.
 17. Apearl lustre pigment according to claim 16, wherein the platelet shapedsubstrate has a thickness between 0.2 and 2 μm and an extent in theother two directions between 2 and 100 μm.
 18. A pearl lustre pigmentaccording to claim 1, wherein the material of high refractive index ischromium oxide, bismuth vanadate, cobalt aluminate, a metal sulphide, ametal nitride, a metal oxynitride or a mixture and/or mixed phase ofthese compounds.